Many buildings have air conditioning or ventilation systems that distribute air throughout the building through ducts connected to diffusers. The diffusers distribute supply air into the spaces to be air conditioned or ventilated. Due to space constraints, such as ceiling grid dimensions into which diffusers may be required to fit, the maximum airflow rate per diffuser is often restricted to a less than optimum value, requiring the added expense of additional diffusers.
Many diffusers incorporate adjustable dampers or adjustable blades for airflow adjustment that provide a generally constant discharge velocity from the diffuser to maintain largely constant throw of the supply air into the occupancy space regardless of the damper or blade airflow setting. These adjustable dampers or blades may be regulated by means of thermally, electrically or pneumatically powered actuators, allowing a degree of individual occupancy space air temperature control to be achieved for the subzone served by that diffuser.
Adjustable blades are sometimes used to alter diffuser discharge direction—manually or by means of thermal, pneumatic or electric actuators. The airflow rate from such diffusers and the position of the diffuser dampers or blades is often affected by supply air pressure fluctuations in the supply duct system, e.g. due to the opening or closing of other dampers. This often results in poor temperature control of the subzones in question as the airflow rate discharged by each diffuser increases or decreases due to the increased or decreased supply air pressure, respectively, and due to further opening or closing of the diffuser's adjustable damper or adjustable blades caused by the elasticity of the damper/blade mechanism.